H-Aggregated Ï-Systems Based on Disulfide-Linked Dimers of

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Cite This: J. Org. Chem. 2017, 82, 11166-11172

H‑Aggregated π‑Systems Based on Disulfide-Linked Dimers of Dipyrrolyldiketone Boron Complexes Yoshifumi Sasano,† Ryuma Sato,‡ Yasuteru Shigeta,‡ Nobuhiro Yasuda,§ and Hiromitsu Maeda*,† †

Department of Applied Chemistry, College of Life Sciences, Ritsumeikan University, Kusatsu 525−8577, Japan Center for Computational Sciences, University of Tsukuba, Tsukuba 305−8577, Japan § Research and Utilization Division, Japan Synchrotron Radiation Research Institute, Sayo 679−5198, Japan ‡

S Supporting Information *

ABSTRACT: Dipyrrolyldiketone boron complexes linked by a disulfide bond were synthesized, forming H-aggregated dimers assisted by intramolecular π−π and hydrogen-bonding interactions. The conformations of the dimers, with small C−S−S−C dihedral angles, were examined by UV−vis absorption and 1H NMR spectra as well as single-crystal X-ray analysis and theoretical studies.

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INTRODUCTION

π−π Stacking interaction plays a crucial role in supramolecular assemblies, organic electronic materials, biological systems, etc.1 and can control electronic properties according to the relative arrangement of the constituting π-electronic systems. A strategy for the construction of effective and tunable π−π interactions is to connect multiple π-electronic systems by covalent linkages, enabling the formation of intramolecular interactions.2 Dimers of π-electronic systems can provide characteristic properties compared to corresponding monomers due to their closepacked π-systems, such as photochromisms,2a molecular actuators,2b,c chiral assembled systems,2d excimer emissions,2c,f and switching aromatic and antiaromatic characters.2g As fascinating π-electronic systems, 1,3-dipyrrolyl-1,3-diketone boron complexes 1a,b show high anion-binding abilities to form diverse anion complexes and can thus afford ion-pairing dimension-controlled assemblies by combining with appropriate cations (Figure 1a).3 Interesting properties are observed in several derivatives, whose modifications have been carried out by the introduction of substituents at the pyrrole α- and βpositions,3,4 meso-position (2-position of the 1,3-propanedione unit)5 (Figure 1b), and the boron unit.6 The dimers of dipyrrolyldiketone BF2 complexes, showing high anion-binding abilities compared to monomers, formed anion-binding helical structures as ion pairs with a chiral cation, exhibiting high performance in circular dichroism and circularly polarized luminescence.7 Based on the background, we endeavored the dimerization of dipyrrolyldiketone boron complexes by the covalent linkage between meso-positions for inducing intramolecular π−π interaction. Disulfide bonds are sufficiently long © 2017 American Chemical Society

Figure 1. (a) Anion-binding mode of dipyrrolyldiketone boron complexes 1a,b; (b) meso-substituted dipyrrolyldiketone boron complexes 2a−c; and (c) overlapped areas of meso-linked and α- or β-linked dipyrrolyldiketone boron complex dimers.

(ca. 2.0 Å) and flexible, as seen for previously reported πsystems,8,9 and hence were chosen for linking the π-electronic systems. Furthermore, the overlapped areas of the π-systems in meso-linked dimers should be larger than those in α- or β-linked dimers, particularly because of the increased dihedral angle θ values (Figure 1c). Consequently, π−π interaction would be more effective in meso-linked dimers than in α- or β-linked dimers. Although most disulfide bonds show θ values around 90°−100°,8 appropriate interactions such as hydrogen bonding10 can achieve smaller θ for efficient π−π stacking. Received: August 30, 2017 Published: September 29, 2017 11166

DOI: 10.1021/acs.joc.7b02185 J. Org. Chem. 2017, 82, 11166−11172

Article

The Journal of Organic Chemistry

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The 1H NMR chemical shifts of the pyrrole units in CDCl3 were observed at 10.01, 8.11, 7.25, and 6.50 ppm for 6a and 9.82, 7.90, 7.23, and 6.43 ppm for 7a, whereas the UV−vis absorption spectra in CH2Cl2 exhibited maxima (λmax) at 440 and 430 nm for 6a and 7a, respectively (Figure 3). The blue-

RESULTS AND DISCUSSION Synthesis and Characterization. For the preparation of meso-disulfide-linked dimers of dipyrrolyldiketone boron complexes (Figure 2), introduction of a chlorine moiety at

Figure 3. UV−vis absorption spectra of 6a (red) and 7a (blue) in CH2Cl2 (1.0 × 10−5 M) at 20 °C and single-crystal X-ray structure of 6a (inset).

shifted λmax of 7a compared to that of 6a and also 5a (439 nm), along with fluorescence quenching, suggested the formation of H-aggregated dimers (closed dimers) in solution state. According to the Kasha’s exciton theory,14 an excitation with the energy level E splits to those with E′ and E″ (E′ < E″). The excitation with E′ becomes forbidden when two chromophores oriented in parallel, whereas it becomes allowed when two chromophores were orthogonally arranged. The shoulder band, for example, at 465 nm of 7a was derived from the contribution of orthogonally arranged dimers, suggesting the formation of “oblique H-aggregated” dimers with dihedral angles of ca. 30°− 40° in the optimized structures (vide infra). Similar absorption and fluorescence spectra were observed in toluene, THF, MeOH, and acetone, suggesting that both intramolecular π−π and N−H···F hydrogen-bonding interactions effectively promoted the formation of H-aggregated dimers (Supporting Figures 16 and 17). Conformations in Solution State. Coalesced 1H NMR signals of 7a in CD2Cl2 at 20 °C were observed probably due to fast equilibria between several conformations and were separated upon cooling from 20 to −80 °C. The 1H NMR at −80 °C showed the existence of a pyrrole-inverted conformation along with the pyrrole-noninverted form (Figure 4a). DFT calculations at the B3LYP-GD3BJ/6-31G(d,p) level15 showed small energy differences between several pyrroleinverted conformations, also suggesting the existence of such pyrrole-inverted conformations in solution state. The 1H NMR signals of 7a at −80 °C could be ascribable to coexisting two conformations, pyrrole-noninverted 7a-1(i) and singly pyrroleinverted 7a-2(i) (Supporting Figure 38; the detail of the former is discussed in the following paragraphs), in the ratio of 1:0.48. The major pyrrole-NH signal (H1) of 7a-1(i) was observed at 9.70 ppm (4H) as a coalesced signal of two kinds of NH by fast exchanges through the rotation around the S−S bond. The minor pyrrole-NH signals (H′1a, H′1b + H′1c, H′1d) of 7a-2(i) were observed at 10.98 (1H), 9.87 (2H), and 9.49 (1H) ppm, respectively, based on the consideration of the downfield shifted H′1a and H′1b + H′1c due to the deshielding effects by meso-sulfur moiety and intramolecular N−H···F hydrogen bonding, respectively. Furthermore, the major pyrrole-CH signal (H2, H3, and H4) of 7a-1(i) were observed at 8.01 (4H),

Figure 2. Synthetic route for meso-disulfide-linked dimers of dipyrrolyldiketone boron complexes 7a,b.

the meso-position of dipyrrolyldiketones 1a′,b′4a,d,11 was achieved by using oxone and NH4Cl,12 affording meso-chlorosubstituted 3a,b in 38% and 73% yields, respectively. Then, an acetylthio group was introduced by the treatment of 3a,b with thioacetic acid and triethylamine13 to afford meso-acetylthiosubstituted 4a,b quantitatively and in 73% yield, respectively. Diketones 4a,b were further converted into the corresponding BF2 complexes 5a,b in 60% and 72% yields, respectively, by treating with BF3·OEt2. The obtained π-electronic molecules were characterized by 1H and 13C NMR and MALDI-TOF- and ESI-TOF-MS. 1H NMR chemical shifts of the pyrrole units in 5a, as an example, were observed at 7.76 (β), 7.25 (α), and 6.44 (β) ppm in CDCl3, which were different from the chemical shifts observed for 1a (7.21 (α), 7.17 (β), and 6.44 (β) ppm) and meso-acetoxy-substituted 2b (7.25 (α), 7.15 (β), and 6.47 (β) ppm),5b suggesting a deshielding effect on β-H by the mesosulfur moiety (Supporting Figure 11). A similar downfield shift of the β-H was observed for 5b (Supporting Figure 12). UV− vis absorption spectra of 5a,b in CH2Cl2 exhibited absorption maxima (λmax) at 439 and 506 nm, respectively, which are comparable to those observed for α-unsubstituted 1a (432 nm) and 2b (446 nm) and α-phenyl-substituted 1b (500 nm) and 2c (514 nm). An effective strategy for the synthesis of meso-disulfide-linked dimers of dipyrrolyldiketones is the oxidative coupling of thiol moieties. Thus, 5a,b were treated with K2CO3 for the hydrolysis of the acetylthio group. However, instead of the desired 6a,b, disulfide-bridged dimers 7a,b were directly obtained in 50% and 25% yields, respectively. Dimers 7a,b were successfully converted to 6a,b in 43% and 72% yields, respectively, by reduction with dithiothreitol (DTT) (Figure 2). In rare cases, 6a,b were isolated from the reaction mixtures of 5a,b and K2CO3. 11167


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energies of 8.63 and 7.51 kcal/mol, respectively, compared to the optimized structure with the dihedral angle of 39.4°. The local energy minimum conformation was optimized with the C−S−S−C dihedral angle of 108.6° (iii in Figure 4b) and is close to the general C−S−S−C dihedral angles (90°−100°) but is less stable because of the absence of intramolecular hydrogen bonding. TD-DFT-based theoretical spectrum of 7a (Supporting Figure 51) as the optimized structure at energy minimum (i in Figure 4b)15 was correlated with the observed spectra in CH2Cl2 and other solvents, also suggesting the formation of H-aggregated dimers in solution state. Further increasing dihedral angles led to less stable conformations, as seen at 180° (iv in Figure 4b) with an energy of 17.36 kcal/mol. Furthermore, several conformations with pyrrole inversion could not form efficient intramolecular hydrogen bonding, as evidenced in the theoretical results, which relate the stability with the C−S−S−C dihedral angles (Supporting Figures 38 and 39). In contrast, 7b with the C−S−S−C dihedral angle of 34.0° in the most stable structure did not show a local energy minimum around 100°, indicating that the introduction of phenyl groups enhanced the intramolecular π−π interaction to form H-aggregated dimers (Supporting Figures 40 and 41). Solid-State Assembled Structures. For further examinations of molecular conformations, the solid-state structures of 7a,b (including two pseudopolymorphs, 7b-plate and 7bneedle: the former was obtained in the presence of tetrabutylammonium chloride) were revealed by single-crystal X-ray analysis (Figure 5).17 Single crystals of these dimers were obtained by the vapor diffusion of n-hexane into a CH2Cl2 solution. In 7a, the distance between two sulfur atoms was 2.08 Å, suggesting the formation of a disulfide bond. The C−S−S− C dihedral angle was 90.3°, different from that in the optimized structure. Columnar structures were formed through intramolecular and intermolecular π−π interactions. The intermolecular distance between planes (a-1) and (a-2) (dipyrrolyldiketone 15 atoms each) was 3.61 Å and that between planes (a-3) and (a-4) (dipyrrolyldiketone 15 atoms each) was 3.45 Å, whereas the intramolecular distance between planes (a-5) and (a-6) (pyrrole 5 atoms each) was 3.22 Å. Intermolecular N− H···F hydrogen bonding with N(−H)···F distances of 2.87, 2.96, and 3.05 Å were formed between the columnar structures (Figure 5a). On the other hand, 7b-plate, with an S−S distance of 2.08 Å, exhibited a C−S−S−C dihedral angle of 86.7°. In this case, columnar structures were formed through intermolecular π−π interactions, with an intermolecular distance of 3.23 Å between planes (b-1) and (b-2) (pyrrole 5 atoms each) and that of 3.53 Å between planes (b-3) and (b-4) (pyrrolyldiketone 10 atoms each). Columnar structures were also formed through intermolecular N−H···F hydrogen bonding with the N(−H)···F distance of 2.99 Å. It is noteworthy that no intramolecular π−π interaction and N− H···F hydrogen bonding were seen in the crystal of 7b-plate (Figure 5b). The typical C−S−S−C dihedral angle observed in 7a, which was metastable in solution, was derived from the crystal packing. Moreover, 7b-plate also showed a typical C−S−S−C dihedral angle despite no existence of the metastable state in the energy diagram. In contrast, 7b-needle exhibited a C−S− S−C dihedral angle of 39.1°, as predicted by the theoretical study, and the S−S distance of 2.16 Å, with the intramolecular N−H···F hydrogen bonding with the N(−H)···F distance of 3.38 Å. Columnar structures were formed based on π−π interactions, supported by intermolecular N−H···F hydrogen

Figure 4. (a) 1H NMR spectrum of 7a in CD2Cl2 at −80 °C; (b) diagram of energy in relation to C−S−S−C dihedral angles in 7a with selected conformations i−iv at B3LYP-GD3BJ/6-31G(d,p), wherein the units in yellow are located in front of those in green and red squares in the diagram show the optimized most stable and metastable structures (i and iii), based on the close structures (black squares) with fixed C−S−S−C dihedral angles.

7.24 (4H), and 6.41 (4H) ppm as coalesced signals. On the other hand, the minor pyrrole-CH signals (four kinds of H′2, H′3, and H′4) should be located at 7.66 and 7.55 ppm as well as at 7.24 and 6.41 ppm (overlapped with major signals) with the integral of 12H. The most stable meso-C−S−S−meso-C (C−S−S−C) dihedral angle16 of 7a in the pyrrole-noninverted conformation as a representative form was estimated as 39.4° by geometrical optimizations at the B3LYP-GD3BJ/6-31G(d,p) level (i in Figure 4b).15 The angle is in contrast to the general disulfide dihedral angle of 90°−100° range.8,9 The distance between the two diketone units, defined by the planes of the pyrrole five atoms and those of diketone five atoms, was 3.30 Å, which was shorter than typical π−π stacking distances. Small dihedral angles were observed due to the effective π−π interaction and the intramolecular N−H···F hydrogen bonding with the N(−H)···F distance of 2.79 Å. The diketone unit of 7a was distorted compared to 6a, as seen in the optimized structure along with the single-crystal X-ray structure (Figure 3),17 with the mean-plane deviations (the 15-atom planes) of 0.10 and 0.17 Å for 6a (optimized structure) and 7a, respectively. The arrangement of two π-electronic systems in the optimized structure was consistent with the observed UV−vis absorption spectrum. In order to compare the energy levels in relation to the C− S−S−C dihedral angles, the structures of 7a, as the pyrrolenoninverted conformation, were optimized by fixing the dihedral angles in 10° increments from 20° to 200° at the B3LYP-GD3BJ/6-31G(d,p) level (Figure 4b).15 Interestingly, upon increasing the dihedral angles, the energy minimum was observed at 40°, and the local energy maximum at 90° (ii in Figure 4b) and minimum at 110° were shown, with relative 11168



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SUMMARY We synthesized meso-disulfide-linked dimers of dipyrrolyldiketone boron complexes. UV−vis absorption spectra and DFT calculations suggested the formation of H-aggregated dimers in solution due to intramolecular synergetic π−π and hydrogenbonding interactions between two covalently linked diketone units. Unusually small C−S−S−C dihedral angles were predicted by theoretical studies and corroborated by singlecrystal X-ray analysis. In addition to the meso-disulfide-linked dimers in this study, covalently α- and β-linked dimers deserve to be synthesized for potential fascinating electronic properties according to the orientations of π-electronic systems. Further functionalization of the dimers for the construction of dimension-controlled assemblies is also underway.

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EXPERIMENTAL SECTION

General procedures. Starting materials were purchased from commercial suppliers and used without further purification unless otherwise stated. NMR spectra used in the characterization of products were recorded on a JEOL ECA-600 600 MHz spectrometer. All NMR spectra were referenced to solvent. UV−vis absorption spectra were recorded on a Hitachi U-3500 spectrometer. Fluorescence spectra were recorded on a Hitachi F-4500 fluorescence spectrometer for ordinary solution. Matrix-assisted laser desorption ionization time-offlight mass spectrometries (MALDI-TOF-MS) were recorded on a Shimadzu Axima-CFRplus. Electrospray ionization mass spectrometry (ESI-MS) was done on a BRUKER microTOF instrument using a ESITOF method. TLC analyses were carried out on aluminum sheets coated with silica gel 60 (Merck 5554). Column chromatography was performed on Wakogel C-300 and Merck silica gel 60 and 60H. 2-Chloro-1,3-dipyrrol-2-yl-1,3-propandione, 3a. According to the literature procedure for the related reactions,12 to a MeOH (7.5 mL) solution of 1,3-dipyrrol-2-yl-1,3-propandione 1a′4a,11 (303 mg, 1.50 mmol) were added oxone (1.02 g, 1.65 mmol) and NH4Cl (88.2 mg, 1.65 mmol). The mixture was stirred under N2 for 6 h. After confirming the consumption of the starting material by TLC analysis, the mixture was separated with CH2Cl2 and water. The organic phase was washed with brine, dried over Na2SO4, and evaporated to dryness. The residue was chromatographed over flash silica gel column (3% MeOH/CH2Cl2) and recrystallized from CH2Cl2/n-hexane to afford 3a (137 mg, 0.581 mmol, 38%) as a white solid. Rf = 0.17 (3% MeOH/CH2Cl2). Mp: 188−190 °C. 1H NMR (600 MHz, CDCl3, 20 °C): δ (ppm) 9.38 (s, 2H, NH), 7.16−7.15 (m, 2H, pyrrole-H), 7.10− 7.09 (m, 2H, pyrrole-H), 6.32−6.30 (m, 2H, pyrrole-H), 5.87 (s, 1H, CH). 13C NMR (151 MHz, CD3CN, 20 °C): δ (ppm) 179.5, 129.7, 128.2, 119.5, 111.9, 62.2. MALDI-TOF-MS: m/z (% intensity): 236.1 (100). Calcd for C11H9ClN2O2 ([M]−): 236.04. HRMS (ESI-TOF: measured in the presence of a small amount of Et3N): m/z: 235.0267. Calcd for C11H8ClN2O2 ([M − H]−): 235.0280. This compound was further characterized as a BF2 complex 3a′ by single-crystal X-ray analysis. Synthesis and detailed characterization of 3a′ will be reported elsewhere. 2-Choro-1,3-di(5-phenylpyrrol-2-yl)-1,3-propandione, 3b. According to the literature procedure for the related reactions,12 to a MeOH (0.32 mL) solution of 1,3-di(5-phenylpyrrol-2-yl)-1,3-propandione 1b′4d (22.2 mg, 0.0630 mmol) were added oxone (42.8 mg, 0.0693 mmol) and NH4Cl (3.71 mg, 0.0693 mmol), and the mixture was stirred under N2 for 7 h. After confirming the consumption of the starting material by TLC, the mixture was separated with CH2Cl2 and water. The organic phase was washed with brine, dried over Na2SO4, and evaporated to dryness. The residue was chromatographed over flash silica gel column (3% MeOH/CH2Cl2) and recrystallized from CH2Cl2/ n-hexane to afford 3b (17.7 mg, 0.064 mmol, 73%) as a yellow solid. Rf = 0.28 (3% MeOH/CH2Cl2). Mp: 211−214 °C. 1H NMR (600 MHz, CDCl3, 20 °C): δ (ppm) 9.60 (s, 2H, NH), 7.58 (d, J = 7.2 Hz, 4H, phenyl-H), 7.43 (t, J = 7.8 Hz, 4H, phenyl-H), 7.35 (t, J = 7.2 Hz, 2H, phenyl-H), 7.25−7.24 (m, 2H, pyrrole-H), 6.60−6.59 (m, 2H, pyrrole-H), 5.90 (s, 1H, CH). 13C NMR (151 MHz, CD3CN,

Figure 5. Solid-state structures of (a) 7a, (b) 7b-plate, and (c) 7bneedle: (i) packing diagrams and (ii) definition of planes. Atom color code: brown, pink, yellow, green, blue, red, and orange refer to carbon, hydrogen, boron, fluorine, nitrogen, oxygen, and sulfur, respectively.

bonding with N(−H)···F distance of 3.26 Å. The intermolecular distance between planes (c-1) and (c-2) (phenylpyrrolyldiketone 16 atoms each) was 3.31 Å, whereas intramolecular distance between planes (b-3) and (b-4) (pyrrolyldiketone 10 atoms each) was 3.25 Å (Figure 5c). 11169


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112.7, 93.5, 30.5. UV−vis (CH2Cl2, λmax[nm] (ε, 104 M−1cm−1)): 506 (15.1). MALDI-TOF-MS: m/z (% intensity): 475.0 (100). Calcd for C25H18BF2N2O3S ([M − H]−): 475.11. HRMS (ESI-TOF): m/z: 475.1101. Calcd for C25H18BF2N2O3S ([M − H]−): 475.1105. This compound was further characterized by single-crystal X-ray analysis. BF2 Complex of 2-Sulfanyl-1,3-dipyrrol-2-yl-1,3-propandione, 6a. To a CH2Cl2/Et3N solution (1.7 mL/17 μL) of BF2 complex of 2,2′dithiobis(1,3-dipyrrol-2-yl-1,3-propandione) 7a (1.40 mg, 2.49 μmol; vide infra) was added DTT (dithiothreitol) (2.10 mg, 13.6 μmol), and the mixture was stirred under N2 for 10 min. The mixture was washed with HCl aq and extracted with CH2Cl2, then washed with brine, dried over Na2SO4, and evaporated to dryness. The residue was chromatographed over flash silica gel column (2% MeOH/CH2Cl2) and recrystallized from CH2Cl2/n-hexane to afford 6a (0.66 mg (including ca. 5% of 7a, which was produced by air oxidation), 2.34 μmol, 43% (based on 1H NMR)) as a yellow solid. Another procedure: To a MeOH solution (1.7 mL) of 5a (26.7 mg, 0.082 mmol) was added K2CO3 (17.2 mg, 0.124 mmol), and the mixture was stirred under N2 for 15 min. After confirming the consumption of the starting material by TLC, HCl aq was added to pH 1. The mixture was washed with water and extracted with CH2Cl2, then washed with brine, dried over Na2SO4, and evaporated to dryness. The residue was chromatographed over flash silica gel column (2% MeOH/CH2Cl2) and recrystallized from CH2Cl2/n-hexane to afford 6a (13.5 mg, 0.048 mmol, 59%) as a yellow solid. Rf = 0.18 (2% MeOH/CH2Cl2). 1H NMR (600 MHz, CDCl3, 20 °C): δ (ppm) 10.00 (s, 2H, NH), 8.11−8.10 (m, 2H, pyrrole-H), 7.26−7.25 (m, 2H, pyrrole-H), 6.50−6.49 (m, 2H, pyrrole-H). 13C NMR (151 MHz, CD3CN, 20 °C): δ (ppm) 172.3, 129.7, 127.9, 126.43, 125.35, 113.7. UV−vis (CH2Cl2, λmax[nm] (ε, 104 M−1cm−1)): 440 (7.2). MALDI-TOF-MS: m/z (% intensity): 281.1 (100). Calcd for C11H8BF2N2O2S ([M − H]−): 281.04. HRMS (ESI-TOF): m/z: 281.0373. Calcd for C11H8BF2N2O2S ([M − H]−): 281.0373. This compound was further characterized by single-crystal X-ray analysis. BF2 Complex of 2-Sulfanyl-1,3-di(5-phenylpyrrol-2-yl)-1,3-propandione, 6b. To a CH2Cl2/Et3N solution (1.0 mL/10 μL) of BF2 complex of 2,2′-dithiobis(1,3-di(5-phenylpyrrolyl)-1,3-propandione) 7b (1.30 mg, 1.50 μmol; vide infra) was added DTT (dithiothreitol) (1.23 mg, 7.97 μmol), and the mixture was stirred under N2 for 10 min. The mixture was washed with HCl aq and extracted with CH2Cl2, then washed with brine, dried over Na2SO4, and evaporated to dryness. The residue was chromatographed over flash silica gel column (CH2Cl2) and recrystallized from CH2Cl2/n-hexane to afford 6b (0.94 mg, 2.16 μmol, 72%) as a red solid. Another procedure conducted for 6a provided a trace amount of 6b. Rf = 0.09 (CH2Cl2). 1H NMR (600 MHz, CDCl3, 20 °C): δ (ppm) 10.13 (s, 2H, NH), 8.18 (s, 2H, pyrrole-H), 7.68 (d, J = 8.4 Hz, 4H, phenyl-H), 7.49 (t, J = 7.2 Hz, 4H, phenyl-H), 7.42 (t, J = 7.2 Hz, 2H, phenyl-H), 6.80−6.79 (m, 2H, pyrrole-H). 13C NMR (151 MHz, CD3CN, 20 °C): δ (ppm) 165.5, 143.1, 130.0, 129.9, 128.0, 127.9, 127.1, 112.8 (two signals ascribable to the carbon of pyrrole, phenyl, or meso position are missing due to the overlap with other signals or overlap with signals of 7b, which was produced by air oxidation). UV−vis (CH2Cl2, λmax[nm] (ε, 104 M−1cm−1)): 508 (11.4). MALDI-TOF-MS: m/z (% intensity): 433.2 (100). Calcd for C23H16B2F2N2O2S ([M − H]−): 433.10. HRMS (ESI-TOF): m/z: 433.0993. Calcd for C23H16BF2N2O2S ([M − H]−): 433.0999. BF2 Complex of 2,2′-Dithiobis(1,3-dipyrrol-2-yl-1,3-propandione), 7a. To a MeOH solution (1.3 mL) of 5a (20.0 mg, 0.062 mmol) was added K2CO3 (12.9 mg, 0.093 mmol), and the mixture was stirred under N2 for 15 min. After confirming the consumption of the starting material by TLC, HCl aq was added to pH 1. The mixture was washed with water and extracted with CH2Cl2, then washed with brine, dried over Na2SO4, and evaporated to dryness. The residue was chromatographed over flash silica gel column (2% MeOH/CH2Cl2) and recrystallized from CH2Cl2/n-hexane to afford 7a (8.90 mg, 0.032 mmol, 50%) as a yellow solid. Rf = 0.22 (2% MeOH/CH2Cl2). Mp: >300 °C (dec). 1H NMR (600 MHz, CDCl3, 20 °C): δ (ppm) 9.82 (s, 4H, NH), 7.90 (s, 4H, pyrrole-H), 7.22−7.20 (m, 4H, pyrrole-H), 6.44−6.42 (m, 4H, pyrrole-H). 13C NMR (151 MHz, CD3CN, 20

20 °C): δ (ppm) 179.0, 141.5, 131.4, 130.6, 129.9, 129.5, 126.7, 121.1, 110.1, 62.2. UV−vis (CH2Cl2, λmax[nm] (ε, 104 M−1cm−1)): 521 (9.6). MALDI-TOF-MS: m/z (% intensity): 388.1 (100). Calcd for C23H17ClN2O3 ([M]−): 388.10. HRMS (ESI-TOF): m/z: 387.0890. Calcd for C23H16ClN2O2 ([M − H]−): 387.0906. 2-Acetylthio-1,3-dipyrrol-2-yl-1,3-propandione, 4a. According to the literature procedure for the related reactions,13 to a CH2Cl2 solution (5.0 mL) of 3a (100 mg, 0.424 mmol) were added thioacetic acid (29.6 μL, 0.424 mmol) and Et3N (58.5 μL, 0.424 mmol) at 0 °C. The mixture was allowed to rt and stirred under N2 for 2.5 h. After the mixture was evaporated, the residue was chromatographed over flash silica gel column (40% EtOAc/n-hexane) to afford 4a (117 mg, 0.424 mmol, quant.) as a brown slurry. Rf = 0.22 (40% EtOAc/n-hexane). 1H NMR (600 MHz, CDCl3, 20 °C): δ (ppm) 9.40 (s, 2H, NH), 7.18− 7.16 (m, 2H, pyrrole-H), 7.09−7.08 (m, 2H, pyrrole-H), 6.34 (s, 1H, meso-H), 6.31−6.30 (m, 2H, pyrrole-H), 2.40 (s, 3H, CH3). 13C NMR (151 MHz, CD3CN, 20 °C): δ (ppm) 194.1, 180.9, 130.5, 128.0, 119.3, 111.8, 56.7, 30.2. MALDI-TOF-MS: m/z (% intensity): 275.0 (100). Calcd for C13H11N2O3S ([M − H]−): 275.05. HRMS (ESITOF): m/z: 275.0496. Calcd for C13H11N2O3S ([M − H]−): 275.0496. 2-Acetylthio-1,3-di(5-phenylpyrrol-2-yl)-1,3-propandione, 4b. According to the literature procedure for the related reactions,13 to a CH2Cl2 solution (3.3 mL) of 3b (100 mg, 0.257 mmol) were added thioacetic acid (21.7 μL, 0.257 mmol) and Et3N (43.0 μL, 0.308 mmol) at 0 °C. The mixture was allowed to rt and stirred under N2 for 2.5 h. After the mixture was evaporated, the residue was chromatographed over flash silica gel column (30% EtOAc/n-hexane) to afford 4b (80.7 mg, 0.188 mmol, 73%) as a brown slurry. Rf = 0.22 (30% EtOAc/n-hexane). 1H NMR (600 MHz, CDCl3, 20 °C): δ (ppm) 9.65 (s, 2H, NH), 7.58 (d, J = 8.4 Hz, 4H, phenyl-H), 7.42 (t, J = 7.8 Hz, 4H, phenyl-H), 7.34 (t, J = 7.2 Hz, 2H, phenyl-H), 6.60−6.59 (m, 2H, pyrrole-H), 2.43 (s, 3H, CH3). 13C NMR (151 MHz, CD3CN, 20 °C): δ (ppm) 194.3, 180.5, 141.3, 131.5, 131.4, 129.8, 129.4, 126.4, 120.9, 109.9, 56.6, 30.2. MALDI-TOF-MS: m/z (% intensity): 427.2 (100). Calcd for C25H19N2O3S ([M − H]−): 427.11. HRMS (ESI-TOF): m/ z: 427.1102. Calcd for C25H19N2O3S ([M − H]−): 427.1122. BF2 Complex of 2-Acetylthio-1,3-dipyrrol-2-yl-1,3-propandione, 5a. To a CH2Cl2 solution (60 mL) of 4a (97.0 mg, 0.351 mmol) was added BF3·OEt2 (496 mg, 3.50 mmol), and the mixture was stirred under N2 for 10 min. After confirming the consumption of the starting material by TLC analysis, the mixture was evaporated. The residue was chromatographed over flash silica gel column (5% MeOH/CH2Cl2) and recrystallized from CH2Cl2/n-hexane to afford 5a (69.0 mg, 0.213 mmol, 60%) as a yellow solid. Rf = 0.33 (5% MeOH/CH2Cl2). Mp: 206−209 °C. 1H NMR (600 MHz, CDCl3, 20 °C): δ (ppm) 9.88 (s, 2H, NH), 7.76−7.74 (m, 2H, pyrrole-H), 7.24−7.23 (m, 2H, pyrroleH), 6.45−6.43 (m, 2H, pyrrole-H), 2.44 (s, 3H, CH3). 13C NMR (151 MHz, CD3CN, 20 °C): δ (ppm) 194.3, 172.5, 130.2, 126.9, 125.5, 114.3, 30.5 (one signal ascribable to the carbon of pyrrole or meso position is missing due to the overlap with other signals). UV−vis (CH2Cl2, λmax[nm] (ε, 104 M−1cm−1)): 439 (9.5). MALDI-TOF-MS: m/z (% intensity): 323.1 (100). Calcd for C13H10BF2N2O3S ([M − H]−): 323.05. HRMS (ESI-TOF): m/z: 323.0459. Calcd for C13H10BF2N2O3S ([M − H]−): 323.0479. This compound was further characterized by single-crystal X-ray analysis. BF2 Complex of 2-Acetylthio-1,3-di(5-phenylpyrrol-2-yl)-1,3propandione, 5b. To a CH2Cl2 solution (40 mL) of 4b (10.8 mg, 0.0250 mmol) was added BF3·OEt2 (35.5 mg, 0.250 mmol), and the mixture was stirred under N2 for 10 min. After confirming the consumption of the starting material by TLC, the mixture was evaporated. The residue was chromatographed over flash silica gel column (30% EtOAc/n-hexane) and recrystallized from CH2Cl2/nhexane to afford 5b (8.6 mg, 0.0180 mmol, 72%) as a yellow solid. Rf = 0.36 (30% EtOAc/n-hexane). Mp: 242−247 °C. 1H NMR (600 MHz, CDCl3, 20 °C): δ (ppm) 9.92 (s, 2H, NH), 7.83−7.82 (m, 2H, pyrrole-H), 7.65 (d, J = 7.2 Hz, 4H, phenyl-H), 7.49 (t, J = 7.8 Hz, 4H, phenyl-H), 7.42 (t, J = 7.8 Hz, 2H, phenyl-H), 6.74−6.73 (m, 2H, pyrrole-H), 2.47 (s, 3H, CH3). 13C NMR (151 MHz, CD3CN, 20 °C): δ (ppm) 194.5, 171.0, 143.1, 130.8, 130.1, 129.9, 128.1, 127.2, 127.0, 11170



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°C): δ (ppm) 172.4, 130.3, 127.4, 126.3, 114.2 (one signal ascribable to the carbon of pyrrole or meso position is missing due to the overlap with other signals). UV−vis (CH2Cl2, λmax[nm] (ε, 104 M−1cm−1)): 430 (9.4). MALDI-TOF-MS: m/z (% intensity): 561.2 (100). Calcd for C22H15B2F4N4O4S2 ([M − H]−): 561.07. HRMS (ESI-TOF): m/z: 561.0661. Calcd for C22H15B2F4N4O4S2 ([M − H]−): 561.0661. This compound was further characterized by single-crystal X-ray analysis. BF2 Complex of 2,2′-Dithiobis(1,3-di(5-phenylpyrrol-2-yl)-1,3propandione), 7b. To a MeOH solution (1.6 mL) of 5b (37.6 mg, 0.079 mmol) was added K2CO3 (13.1 mg, 0.095 mmol), and the mixture was stirred under N2 for 15 min. After confirming the consumption of the starting material by TLC, HCl aq was added to pH 1. The mixture was washed with water and extracted with CH2Cl2, then washed with brine, dried over Na2SO4, and evaporated to dryness. The residue was chromatographed over flash silica gel column (CH2Cl2) and recrystallized from CH2Cl2/n-hexane to afford 7b (8.20 mg, 0.0095 mmol, 25%) as a red solid. Rf = 0.22 (CH2Cl2). Mp: 200 °C (dec). 1H NMR (600 MHz, CDCl3, 20 °C): δ (ppm) 9.70 (s, 4H, NH), 8.04 (d, J = 3.6 Hz, 4H, pyrrole-H), 7.52 (d, J = 7.2 Hz, 8H, phenyl-H), 7.42−7.38 (m, 12H, phenyl-H). 13C NMR (151 MHz, CD3CN, 20 °C): δ (ppm) 148.7, 143.1, 130.8, 130.0, 129.8, 128.6, 127.9, 127.2, 113.8, 112.8 (one signal ascribable to the carbons of pyrrole, phenyl, or meso position was missing due to the overlap with other signals). UV−vis (CH2Cl2, λmax[nm] (ε, 104 M−1cm−1)): 488 (19.1). MALDI-TOF-MS: m/z (% intensity): 889.2 (100). Calcd for C46H32B2F4N4O4S2Na ([M + Na]+): 889.19. HRMS (ESI-TOF): m/z: 865.1903. Calcd for C46H31B2F4N4O4S2 ([M − H]−): 865.1914. This compound was further characterized by single-crystal X-ray analysis. Method for Single-Crystal X-ray Analysis. Crystallographic data are summarized in the Supporting Information and Supporting Table 1. A single crystal of 3a′ was obtained by vapor diffusion of nhexane into a CH2Cl2 solution. The data crystal was an orange prism of approximate dimensions 0.01 mm × 0.01 mm × 0.01 mm. A single crystal of 5a was obtained by vapor diffusion of n-hexane into a CH2Cl2 solution. The data crystal was an orange prism of approximate dimensions 0.27 mm × 0.27 mm × 0.27 mm. A single crystal of 5b was obtained by vapor diffusion of n-hexane into a CH2Cl2 solution. The data crystal was a purple prism of approximate dimensions 0.48 mm × 0.10 mm × 0.10 mm. A single crystal of 6a was obtained by vapor diffusion of n-hexane into a CH2Cl2 solution. The data crystal was a purple prism of approximate dimensions 0.06 mm × 0.02 mm × 0.01 mm. A single crystal of 7a was obtained by vapor diffusion of n-hexane into an CH2Cl2 solution. The data crystal was a yellow prism of approximate dimensions 0.02 mm × 0.02 mm × 0.01 mm. A single crystal of 7b-plate was obtained by vapor diffusion of n-hexane into a CH2Cl2 solution even though 1 equiv of tetrabutylammonium chloride (TBACl) was added. The data crystal was a purple prism of approximate dimensions 0.10 mm × 0.04 mm × 0.02 mm. A single crystal of 7b-needle was obtained by vapor diffusion of n-hexane into a CH2Cl2 solution. The data crystal was a red prism of approximate dimensions 0.01 mm × 0.01 mm × 0.01 mm. Data of 5a, 5b, and 7bplate were collected at 93 K on a Rigaku XtaLAB P200 diffractometer with graphite monochromated Cu−Kα radiation (λ = 1.54187 Å), whereas data of 3a′, 6a, 7a, and 7b-needle were collected at 75 K on a Rigaku Saturn 724 diffractometer with Si (111) monochromated synchrotron radiation (λ = 0.78203 Å) at BL40XU (SPring-8),17 and structures were solved by direct method. In each compound, the nonhydrogen atoms were refined anisotropically. The calculations were performed using the Crystal Structure crystallographic software package of Molecular Structure Corporation.18 CIF files (CCDC 1548615−1548621) can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif. DFT calculations. DFT calculations were carried out using Gaussian 09 program.15

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.joc.7b02185. Spectroscopic data, X-ray crystallographic data, theoretical study (PDF) Crystallographic data (CCDC 1548615−1548621) (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hiromitsu Maeda: 0000-0001-9928-1655 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI grant numbers JP26288042 for Scientific Research (B) and JP26107007 for Scientific Research on Innovation Areas “Photosynergetics” and Ritsumeikan Global Innovation Research Organization (RGIRO) project (2017−2022). Theoretical calculations were partially performed using Research Center for Computational Science, Okazaki, Japan. We thank Prof. Atsuhiro Osuka, Dr. Takayuki Tanaka, Dr. Koji Naoda, Mr. Shinichiro Ishida, and Mr. Takanori Soya, Kyoto University, for single-crystal X-ray analysis for 5a,b and 7b-plate, Prof. Hikaru Takaya, Kyoto University, and Dr. Ryohei Yamakado, Yamagata University, for synchrotron radiation single-crystal X-ray analysis for 6a, 7a, and 7b-needle (BL40XU at SPring-8), and Prof. Hitoshi Tamiaki, Ritsumeikan University, for various measurements.

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REFERENCES

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